This invention relates to compositions of matter and methods of using them to inhibit the polymerization of vinyl aromatic monomers. Many of these monomers undesirably polymerize at various stages of their manufacture, processing, handling, storage, and use. These undesirable polymerization reactions result in a loss in production efficiency because they consume valuable reagents and because they require and additional purification steps to remove the undesired polymers. Undesired polymerization reactions are particularly problematic by vinyl aromatic monomers and form unwanted polymer during the purification process.
Two categories of compounds have been developed to prevent unwanted polymerization reactions, inhibitors and retarders. Inhibitors prevent polymerization reactions from occurring. Inhibitors however are consumed rapidly. In cases of emergency when for a mechanical or other reason more inhibitor cannot be added, previously added inhibitor will be rapidly consumed and the unwanted polymerization reactions will then rapidly recur. Retarders slow down the rate of polymerization reactions but are not as effective or as inhibitors. Retarders however are usually not consumed as quickly so they are more reliable in cases of emergency.
At first only retarders such as sulfur, and dinitrophenols (DNP) (including 2,6-dinitrophenol, 2,4-dinitrocresol, and 2-sec-butyl-4,6-dinitrophenol (DNBP)) were used to prevent unwanted polymerization reactions. Later two classes of inhibitors were used dialkylhydroxylamines (including hydroxypropylhydroxyamine (HPHA)) and nitroxides (so-called stable free radicals). Because of safety concerns in the event of a plant malfunction, inhibitors alone cannot be used and they therefore are often combined with retarders.
DNP retarders however are highly toxic and there is a significant need for a replacement for them. One class of compounds that was hoped can be a retarder in the place of DNP are quinone methides. Quinone methides slow the rate of polymer formation under static conditions and do not need to be frequently re-fed. However quinone methides must be used in fairly high dosages, so they are not very economical to use by themselves. Examples of quinone methide compounds are in U.S. Pat. No. 4,003,800. These compound however are not stable enough for sustained use in industrial settings. Other applications of quinone methides are found in U.S. Pat. Nos. 5,583,247, and 7,045,647.
Previous examples of inhibitor-retarder combinations that do not use DNP are U.S. Pat. Nos. 5,446,220 and 6,024,894. These combinations were found to be more effective than DNP alone. They were however found to be less effective than the previous DNP-nitroxide or DNP-dialkylhydroxylamines combinations. Thus there still remains a need for a non-toxic inhibitor-retarder combination for use in preventing the premature polymerization of styrene and other vinyl aromatic monomers.
The art described in this section is not intended to constitute an admission that any patent, publication or other information referred to herein is “prior art” with respect to this invention, unless specifically designated as such. In addition, this section should not be construed to mean that a search has been made or that no other pertinent information as defined in 37 C.F.R. § 1.56(a) exists. Any and all patents, patent applications, and other references cited by this application are hereby incorporated by reference in their entirety.